(a) Schematic diagram of attachment of kinetochore microtubules to the sister chromatids of a metaphase chromosome. In animals and lower plants, the kinetochore is a three-layer, platelike structure lying within the centromere of each chromatid (inset). The (+) ends of microtubules insert into the outer layer of each kinetochore, and the microtubules extend toward one of the two poles of the cell. At anaphase, the sister chromatids separate, and the chromosomes are pulled to opposite poles of the cell by the kinetochore microtubules. (b) Association of a yeast centromere with components of the kinetochore. The centromeric (CEN) DNA is divided into three contiguous segments (CDE I – III). Two groups of centromere-binding factors, CBF2 and CBF3, are proteins associated with CDE II and CDE III, respectively. The CBFs mediate the attachment of a single microtubule to the centromere. Other microtubule-binding proteins form the rest of the kinetochore. [Part (a) adapted from A. F. Pluta et al., 1995, Science 270:1591.]